concrete functor and monad transformers, each with associated
operations and functions to lift operations associated with other
transformers.

The package can be used on its own in portable Haskell code, in
which case operations need to be manually lifted through transformer
stacks (see Control.Monad.Trans.Class for some examples).
Alternatively, it can be used with the non-portable monad classes in
the mtl or monads-tf packages, which automatically lift operations
introduced by monad transformers through other transformers.